Abstract
Some of the best-performing high-temperature magnets are Sm–Co-based alloys with a microstructure that comprises an
$$\hbox {Sm}_2\hbox {Co}_{17}$$
Sm
2
Co
17
matrix and magnetically hard
...$$\hbox {SmCo}_5$$
SmCo
5
cell walls. This generates a dense domain-wall-pinning network that endows the material with remarkable magnetic hardness. A precise understanding of the coupling between magnetism and microstructure is essential for enhancing the performance of Sm–Co magnets, but experiments and theory have not yet converged to a unified model. Here, transmission electron microscopy, atom probe tomography, and nanometer-resolution off-axis electron holography have been combined with micromagnetic simulations to reveal that the magnetization state in Sm–Co magnets results from curling instabilities and domain-wall pinning effects at the intersections of phases with different magnetic hardness. Additionally, this study has found that topologically non-trivial magnetic domains separated by a complex network of domain walls play a key role in the magnetic state by acting as nucleation sites for magnetization reversal. These findings reveal previously hidden aspects of magnetism in Sm–Co magnets and, by identifying weak points in the microstructure, provide guidelines for improving these high-performance magnetic materials.
In this study the interplay between recrystallization and precipitation in a biodegradable TWIP (twinning-induced plasticity) steel developed for use in temporary implants was investigated. ...Microstructural and mechanical properties were studied and a thermomechanical treatment was designed with the aim of achieving an overall performance suitable for the intended application as temporary implant material. The formation of Pd-rich precipitates in the cold-worked state was found to considerably retard recrystallization during an annealing treatment. The formation, morphology and interaction with dislocations of these precipitates were studied by means of scanning and transmission electron microscopy. Grain boundary pinning by Pd-rich precipitates (Zener drag) and reduced dislocation mobility due to a solute drag effect caused by the enrichment of dislocation cores with Pd were both identified as mechanisms which impede recrystallization. A model is reported which explains the interplay between recrystallization and precipitation, and provides the basis for the optimized thermomechanical treatment then presented. The resulting mechanical properties, in particular the combination of high strength and ductility with a pronounced strain-hardening response, exceed the performance of other TWIP steels and alloys typically used in biomedical implants, such as stainless steel, titanium or cobalt–chromium alloys. The specific property profile developed is especially advantageous for the production and deployment of cardiovascular stents.
Detailed microscopic observations of the shear surfaces of a deformed, but unfractured, bulk metallic glass sample reveal a wealth of information on the deformation characteristics, kinetics and ...influence of temperature during serrated flow. The shear surfaces exhibit shear striations, which are similar to those resulting from viscous-like flow in rock-forming minerals. On the shear surface only a few areas show typical vein patterns, the thicknesses of which are less than those known from fracture surfaces. Combined with estimates for adiabatic heating, this indicates that sufficiently high temperatures are already present during shear banding before fracture, though instigated by non-purely adiabatic effects. A kinetic model based on an energy variable which reflects the structural relaxation ability is proposed that accounts for the occurrence of serrated flow combined with negative strain rate sensitivity, and the transition to non-serrated flow, i.e. positive strain rate sensitivity, below a critical temperature and strain rate.
This study investigates the ternary intermetallic phases in the Mg–Zn–Ca system, which is of great interest for metallic biodegradable implant applications. According to published phase diagrams, the ...key alloy composition studied herein is located within the Ca2Mg5Zn5, Ca2Mg6Zn3, and IM1 phase fields. Through controlled cooling of the melt, a quasibinary ∼Ca2Mg5Zn5–Mg microstructure was obtained. The large polygonal grains had a composition of Ca2Mg5Zn5 as determined by energy-dispersive x-ray spectroscopy (EDX). Differential scanning calorimetry revealed that Ca2Mg5Zn5 begins to form at ∼417 °C, and the eutectic temperature is ∼369 °C. Based on single-crystal x-ray diffraction data, Ca2Mg5Zn5 was determined to be hexagonal (P63/mmc), with lattice parameters of a = 9.5949(3) Å and c = 10.0344(3) Å. This was also verified by transmission electron microscopy. Further refinements, which considered the possibility of mixed Mg/Zn sites, significantly improved the data fit compared to the initial ordered structural model. The final refined structure possesses a composition of Ca16Mg42Zn42, very similar to the chemical analysis results from EDX.
Magnesium (Mg) alloys have attracted great attention as potential materials for biodegradable implants. It is essential that an implant material possesses adequate resistance to cracking/fracture ...under the simultaneous actions of corrosion and mechanical stresses, i.e., stress corrosion cracking (SCC) and/or corrosion fatigue (CF). This study investigates the deformation behaviour of a newly developed high-strength low-alloy Mg alloy, MgZn1Ca0.3 (ZX10), processed at two different extrusion temperatures of 325 and 400°C (named E325 and E400, respectively), under slow strain tensile and cyclic tension-compression loadings in air and modified simulated body fluid (m-SBF). Extrusion resulted in a bimodal grain size distribution with recrystallised grain sizes of 1.2 μm ± 0.8 μm and 7 ± 5 μm for E325 and E400, respectively. E325 possessed superior tensile and fatigue properties to E400 when tested in air. This is mainly attributed to a grain-boundary strengthening mechanism. However, both E325 and E400 were found to be susceptible to SCC at a strain rate of 3.1×10−7s−1 in m-SBF. Moreover, both E325 and E400 showed similar fatigue strength when tested in m-SBF. This is explained on the basis of crack initiation from localised corrosion following tests in m-SBF.
Magnesium-based bulk metallic glasses (BMGs) exhibit high specific strengths and excellent glass-forming ability compared to other metallic systems, making them suitable candidates for ...next-generation materials. However, current Mg-based BMGs tend to exhibit low thermal stability and are prone to structural relaxation and brittle failure. This study presents a range of new magnesium-precious metal-based BMGs from the ternary Mg-Ag-Ca, Mg-Ag-Yb, Mg-Pd-Ca and Mg-Pd-Yb alloy systems with Mg content greater than 67 at.%. These alloys were designed for high ductility by utilising atomic bond-band theory and a topological efficient atomic packing model. BMGs from the Mg-Pd-Ca alloy system exhibit high glass-forming ability with critical casting sizes of up to 3 mm in diameter, the highest glass transition temperatures (>200 °C) of any reported Mg-based BMG to date, and sustained compressive ductility. Alloys from the Mg-Pd-Yb family exhibit critical casting sizes of up to 4 mm in diameter, and the highest compressive plastic (1.59%) and total (3.78%) strain to failure of any so far reported Mg-based glass. The methods and theoretical approaches presented here demonstrate a significant step forward in the ongoing development of this extraordinary class of materials.
High-performance ultra-lean binary Mg–Ca alloys are engineered by intelligent alloying and thermo-mechanical processing using hot-extrusion. With Ca-alloying contents as low as 0.2-0.6 wt.%, ...remarkable room-temperature tensile properties are obtained with tensile strength values as high as 380–420 MPa, or ductility values reaching a maximum of 36 %. By means of multiscale structural and chemical analysis using electron microscopy and energy dispersive X-ray spectroscopy, we show that multimodal strengthening mechanisms can be activated by modifying the spatial distribution of Ca as secondary phase. Our results indicate that strong precipitation strengthening is achieved when Mg2Ca phase particles are dispersed within the grains. On the other hand, preferential distribution of the Mg2Ca precipitates along grain boundaries imparts substantial grain-boundary strengthening by the Hall-Petch effect. Apart from secondary-phase precipitation, the role of Ca as solute atoms is paramount in promoting homogeneous deformation. The presence of Ca directly alters the intrinsic stacking fault energies and modifies the cross-slip energy barriers such that the slip-transition probability from pyramidal-to-basal and vice-versa becomes comparable. Both effects ensure competitive activation of basal and non-basal slip, thereby reducing the mechanical anisotropy. The mechanical performance in the current work, when compared to earlier reported studies of Mg alloys with similar or higher alloying content, shows a 2 to 10-fold increase in tensile strength without compromising ductility.
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The Besnus Transition in Single‐Domain 4C Pyrrhotite Koulialias, Dimitrios; Lesniak, Barbara; Schwotzer, Matthias ...
Geochemistry, geophysics, geosystems : G3,
November 2019, 2019-11-00, 20191101, Letnik:
20, Številka:
11
Journal Article
Recenzirano
The discontinuous change in magnetic properties at a temperature of about 32 K, denoted as the Besnus transition, is a diagnostic feature for detecting 4C pyrrhotite in geological systems. The ...transition is grain size dependent, and it has been assumed that the single‐domain (SD) to multidomain (MD) threshold is in the micron size range. Here we present a combined crystallographic and magnetic study of a pure phase, SD 4C pyrrhotite that was produced by ball milling of a MD precursor. The mechanical treatment generated varying grain sizes of less than 20 μm, which are solely in a SD magnetic state. It also decreases the coherent scattering domains of the grains from 216 nm in the MD sample to 30 nm, which is taken to set limits for the MD/SD threshold in 4C pyrrhotite grains. The alternating current susceptibility response associated with the Besnus transition is diagnostic for distinguishing SD and MD 4C pyrrhotite. The unambiguous identification of SD 4C pyrrhotite provides a better insight into its occurrence in geological systems.
Key Points
A combined crystallographic and magnetic approach is presented to identify and characterize single domain 4C pyrrhotite
The multi‐ to single‐domain threshold in 4C pyrrhotite is attributed to the coherent scattering domains in a range between 220 and 30 nm
The dynamic magnetic response of the Besnus transition is a diagnostic tool to identify single‐domain 4C pyrrhotite in geological systems
This study presents results on the mechanical properties of Zr-based bulk metallic glasses over a wide range of temperatures (77–673
K) and strain rates (3.3
×
10
−5 − 0.2
s
−1), and provides novel ...information on the kinetics of inhomogeneous deformation in metallic glasses. The disappearance of serrated flow, i.e. temporal homogenization, below a critical temperature and above a critical strain rate corresponds to a transition of the asymptotic strain rate sensitivity (SRS) from negative to positive values. Based on thermally activated shear processes a temperature and strain rate dependent deformation model is presented that includes a time-varying state variable which represents the local degree of relaxation in the shear band after a shear event has taken place. The model makes possible a theoretical description of the observed SRS and the spatio-temporal evolution of plastic deformation in metallic glasses.
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